Structural materials may generally be classified as traditional materials or advanced composites. Whether traditional or advanced, it is important that methods be developed for reliably establishing their elastic properties. Such data are necessary for design, quality control, and in-service evaluation. The determination of elastic properties is more difficult for advanced composites than for traditional materials because traditional materials tend to be homogeneous and isotropic, while composites tend to be non-homogeneous and anisotropic. The isotropic material can be considered as a special case of the more general anisotropic material.
The most widely used composite materials are made of polymer resins reinforced with high strength, high stiffness fibers. Experience has shown that many thin, laminated composite plates, shells and panel structures are essentially orthotropic. Orthotropic materials have three mutually perpendicular planes of material property symmetry. However, some composites are comprised of randomly distributed fibers that result in in-plane properties which are nearly isotropic. That is, all planes are planes of material property symmetry.
The elastic behavior of thin orthotropic composite panels may be adequately described by four elastic constants--the longitudinal Young's modulus, E.sub.x ; the transverse Young's modulus, E.sub.y ; the in-plane shear modulus, G.sub.xy ; and the major Poisson's ratio, V.sub.xy. Isotropic composites may be characterized by such elastic constants as the Young's Modulus, E; and the shear modulus, G.
Presently, a number of standards stipulate methods for measurement of the elastic constants of fiber composites. Among these are the standards for high modulus composites drawn up by the ASTM (1987). Some industrial standards also exist. For example, the three major United States automotive manufacturers have developed special standards for automotive composites (Automotive Composites Consortium, 1990). All these methods are based on static tests. They share the major drawback of involving many samples and require special test fixtures. These methods are consequently slow and expensive.
Against this background, it would be desirable to have a test method that would be useful for quickly and continuously monitoring and controlling the quality of composite components.
Some attempts to derive elastic constants have been based on vibration response data. The present invention, however, differs from previous approaches both in the vibrational deflection approximation and the methodology of obtaining optimum values of the desired elastic constants once the vibrational frequency equation is obtained.